The present invention concerns a spacer for a nuclear boiling water reactor, comprising a plurality of cells for holding parallel elongated elements extending through the cells or for allowing said elements to pass through the cells, wherein a plurality of flow channels are defined between the cells, which flow channels allow a cooling medium to flow therethrough, wherein when the spacer is in use in a nuclear boiling water reactor, the elongated elements extend in an essentially vertical direction and the flow of the cooling medium takes place mainly in a vertical direction upwards, which spacer comprises a plurality of deflecting members which are arranged to deflect the flow of the cooling medium. The invention also concerns a fuel assembly for a nuclear boiling water reactor.
When in the present description and in the following claims flow channels are mentioned in connection with spacers, it is not only referred to the flow channel which is formed in the spacer itself but also to the axial extension of this flow channel, i.e., when the spacer is arranged in a fuel assembly, the flow channels which are formed between the fuel rods.
A boiling water reactor comprises normally a number of fuel assemblies. A fuel assembly comprises a number of elongated fuel rods. FIG. 1 shows an example of a fuel assembly. The fuel assembly is here generally indicated by 10 and the fuel rods are indicated by 12. In order to hold the fuel rods in position and at predetermined distances from each other, the fuel assembly comprises a number of spacers, which in FIG. 1 are indicated by 14. The spacers may have different shapes. FIG. 2a and FIG. 2b show two examples of how a spacer may be designed. A spacer thus comprises a plurality of cells for holding elongated elements, for example fuel rods or guide tubes for control rods, which extend through the cells. Between the cells a number of flow channels 18 extend, which flow channels allow a cooling medium, usually water, to flow therethrough. The fuel assemblies are usually arranged vertically in the nuclear boiling water reactor. Also the fuel rods are thus arranged vertically. The fuel assembly is open in both ends so that the cooling medium may flow through the fuel assembly. The cooling medium thereby usually flows upwards.
In a boiling water reactor, steam is formed close to the fuel rods. Thereby the cooling of the fuel rods is impaired. In order to improve the cooling it is already known that the spacer comprises a number of deflecting members which are arranged for deflecting the flow of the cooling medium. Examples of such deflecting members may be found in SE-C2-503 776. This document shows that a number of deflecting members are arranged such that they are surrounded by the cells and separated axially in order to step by step deflect and control at least a part of the flow of the cooling medium towards the fuel rods. The deflecting members are centrally arranged in the flow channels and have an almost triangular shape and are narrower in their upper part.
There also exists another kind of spacer (a so-called intermediate spacer) which does not have the purpose of holding the fuel rods, but which only has the purpose of holding the deflecting members in position in the flow channels between the fuel rods. Such a kind of spacer comprises cells which allow the fuel rods to pass through the cells without holding them. When xe2x80x9cspacerxe2x80x9d is mentioned in this description and in the following claims, this expression also includes so-called intermediate spacers.
In the previously known spacers, the deflecting members are normally symmetrically arranged in the flow channels. By symmetrically arranged in the flow channel is ment, for example, that if the flow channel is limited by four sides, then there are two or four deflecting members arranged in the flow channel. If there are two deflecting members, then these are arranged on opposite sides. If there are four deflecting members, then theses are arranged on one side each of the four sides which limit the flow channel. Furthermore, the angle of inclination and the size of the different deflecting members are the same such that symmetry is achieved in the device.
In previously known spacers with deflecting members, the deflecting members cause a relatively uncontrolled turbulence in the flow of the cooling medium. This turbulence may lead to an unwanted, relatively high, pressure drop in the reactor.
Different deflecting members are also known in connection with pressure water reactors. For example U.S. Pat. No. 5,440,599 shows deflecting members for a pressure water reactor. The deflecting members extend from the middle of the flow channels and are directed towards the fuel rods. A pressure water reactor has a completely different principle of operation than a boiling water reactor. Inter alia, in a pressure water reactor no (or minimal) steam generation takes place. The spacer construction in a pressure water reactor is thus normally not directly applicable to a boiling water reactor.
The purpose with the present invention is to achieve an improved, controlled deflection of the cooling medium towards the fuel rods, and to avoid unwanted formation of turbulence which may impair the cooling and which causes a pressure drop in the boiling water reactor.
This purpose is achieved with the spacer as initially defined in that a plurality of said deflecting members comprises a vane which extends in a direction from a cell into the neighbouring flow channel, wherein the vane is inclined relative to a vertical plane and wherein the vane is wider in its upper part that in its lower part.
In the upper part of the fuel assembly where a considerable amount of the cooling medium (water) has been converted into steam, the remaining part of the cooling medium exists as a film on the surfaces of the fuel rods and as drops in the steam flow. If the film of the cooling medium is not maintained, an isolating steam layer is formed which leads to a quick increase in temperature and to a destroyed enclosure of the fuel, a so-called dryout. A way of improving the film of the cooling medium with the help of drops in the steam flow improves the margin against dryout and has a considerable economical value and a considerable value for safety reasons.
It has become apparent that with the previously known spacers, different vortices may be formed at different edges of the projecting deflecting members or at different deflecting members in the flow channel. These different vortex formations may disturb or act against each other. This may lead to the effect that the cooling medium is not efficiently deflected towards the fuel rods. The steam which is formed around the fuel rods is therefore not substituted by the cooling medium. Steam therefore stays around the fuel rods which, as has been described above, leads to an increased risk for a dryout. That is, the dryout performance of the reactor becomes worse. Furthermore, the different vortex formations may lead to an increased pressure drop in the reactor. These problems with previously known spacers are thus solved by the present invention. By the invention, an efficient deflection of the cooling medium towards the fuel rods is thus achieved. The formed steam will therefore be concentrated to the middle of the flow channels. An efficient cooling of the fuel rods is thereby achieved and the dryout performance of the reactor is improved.
The vane according to the present invention is preferably formed such that it creates a controlled vortex formation in the flow of the cooling medium, which formation is relieved from the vane at the upper part of the vane.
According to an embodiment of the invention, the deflecting member comprises a plane or some what bent base portion which has an extension in a vertical direction, wherein the vane and the base portion form a continuous unit and the vane is folded-out from the base portion and thus meets the base portion along a line. By such a construction, the deflection member may be produced in a relatively simple manner, since it is folded-out from a base portion.
According to a further embodiment, the base portion constitutes a continuous unit together with the cell, wherein the vane is thus formed as a folded-out portion of the cell. By this embodiment, the deflecting member in the form of a vane may thus be produced in a relatively simple manner, since the deflecting member is formed as a part of the cell which holds the fuel rods in position.
According to a further embodiment, the vane is folded-out such that it forms an angle of about 75-120 degrees with the base portion. By this embodiment, it has become clear that a construction is achieved which has favourable flow properties.
According to a further embodiment, the base portion is configured and arranged such that, when the spacer is positioned for use in a nuclear boiling water reactor, it extends parallel to one of said neighbouring elongated elements. Since the base portion extends parallel to a neighbouring element, for example to a fuel rod, it has become clear that unwanted disturbing turbulence formations close to the base portion may be avoided.
According to a further embodiment of the invention, the vane extends towards and reaches about the middle of the flow channel. Hereby a favourable vortex formation is achieved around the middle of the flow channel such that the cooling medium is deflected out towards the neighbouring fuel rods.
According to another embodiment, the vanes are positioned asymmetrically in the flow channels. By this embodiment, a particularly favourable controlled vortex formation is achieved. It has thus become apparent that the vortices which are formed by symmetrically arranged vanes often disturb each other. The inventor of he present invention has thus realised that an asymmetrical positioning of the vanes in the flow channels leads to a more advantageous and more controlled vortex formation. It has thus become apparent that a controlled symmetrical vortex formation is obtained if the vanes are asymmetrically positioned in the flow channels.
According to another embodiment, there is only one vane per flow channel. The arrangement of only one vane per flow channel is thus an example of an asymmetrically arranged vane. Thereby a controlled vortex formation is achieved, which formation is not disturbed by vortices from any neighbouring deflecting members. It has thus become apparent that by arranging only one vane per flow channel, a controlled symmetrical vortex is obtained, which leads to an even and efficient cooling of the neighbouring fuel rods. At the same time a disturbing turbulence, which may lead to an unwanted pressure drop in the flow of the cooling medium through the fuel assembly, is avoided. A further advantage with only having one vane per flow channel is that it only requires a low amount of additional material in the spacer, and the process of production is simplified as well.
According to a further embodiment of the invention, the vane is plane. Such a vane is relatively easy-to produce and has also good vortex formatting properties.
According to a further embodiment of the invention, the vane is bent. Since the vane is somewhat bent, an advantageous vortex formation may be obtained without leading to a high pressure drop.
According to a further embodiment of the invention, the vane has a first edge which extends in at least a vertical direction and which is closer to the cell in its lower part than in its upper part and a second edge which extends in at least a horizontal direction and which meets that first edge in a corner portion. Hereby a relatively simple vane construction which works well is achieved.
According to a further embodiment of the invention, the first and the second edges are straight. Hereby a relatively easily producible vane with good vortex formation properties is achieved.
According to a further embodiment of the invention, the first or the second or both edges are bent. By a slight bend of the edges, the vane may be optimised from a flow technical point of view.
According to a further embodiment of the invention said corner portion is located at approximately the same level as or higher up than the portion of the second edge which is closer to the cell, such that a vortex formed by the vane in the flow of the cooling medium is relieved from the vane primarily at said corner portion. Hereby vortices acting against each other is avoided, which vortices could create an unwanted and uncontrolled turbulence in the flow channel.
According to a further embodiment, the vane is inclined between 5 and 30 degrees relative to a vertical plane. Such an inclination of the vane has been shown to lead to an advantageous vortex formation in the flow channel.
According to a further embodiment of the invention, the vane in a vertical direction reaches higher than the cells. Since the vane reaches higher than the cells, the vane is not much disturbed by the flow which is influenced by the structure of the spacer. Thereby a longer lasting vortex is formed by the vane. The vortex thus lasts a longer distance up in the flow channel, which leads to an improved cooling of the fuel rods.
According to a further embodiment of the invention, the vane in a vertical direction has an extension, which neither reaches essentially above or essentially below the cells. Preferably, the vane has an extension such that it does not reach above or below the cells at all. The vane is thus in this case formed completely within the extension of the spacer in a vertical direction. This construction has mechanical advantages. The vane may thus in this case easily be formed by a folded-out portion of the cells. Furthermore, the vane is arranged protected in the spacer such that it may not be damaged when the fuel rods are positioned in the spacer. At the same time also the fuel rods are protected from being damaged by a protruding vane when they are put in position in the spacer.
The invention also concerns a fuel assembly for a nuclear boiling water reactor, wherein the fuel assembly comprises a plurality of spacers according to any one of the proceeding embodiments. Thereby a fuel assembly is achieved which has the advantages which have been described above in connection with the embodiments of the spacer.
The invention also concerns a fuel assembly according to claim 19. By this embodiment an advantageous deflection of the flow of the cooling medium may be achieved even in flow channels which are not necessarily located in close connection to a spacer.